The present invention relates to a dielectric filter, and more particularly to the arrangement of coupling adjustment holes in a comb-line type dielectric filter.
Heretofore there has been proposed a comb-line type dielectric filter in which a plurality of holes are made in a dielectric block and the interior surfaces of the holes and other required surface portions of the block are covered with conductor layers to constitute resonators of resonance frequencies dependent on the depths of the holes, as set forth in U.S. Pat. No. 3,505,618. As compared with a conventional filter formed by arranging a plurality of semi-coaxial or coaxial type resonators in a metal case, the comb-line type dielectric filter is highly advantageous in that it is small, lightweight and stable mechanically and electrically. On the other hand, the comb-line type dielectric filter is defective in that adjustment of the resonance frequencies of the coupling between adjacent ones of the resonators is difficult because electromagnetic fields are concentrated in the block as the dielectric constant increases. Especially the coupling adjustment is almost impossible to preform. For this reason, many efforts have been made to eliminate the necessity of adjustment of the filter by reducing dispersion in the quality of the dielectric material used and in improving the working accuracy of the material. As a result of this, the dielectric material becomes very expensive, and hence is not suitable for mass production.
As a solution to such a problem, there has been recently proposed in U.S. patent application Ser. No. 279,461 filed on July 1, 1981, now U.S. Pat. No. 4,410,868, such a filter structure that coupling adjustment holes, not covered with the conductor layer, are formed in the block intermediate between adjacent ones of the resonator holes, thereby to facilitate the coupling adjustment. With the provision of such coupling adjustment holes, however, coupling between adjacent resonators markedly increases. The reason is that the distance (electrical length) between the resonators decreases equivalently as a result of the removal of the dielectric from the portion where each coupling adjustment hole is formed. When the coupling intensity increases, the resonator holes must be spaced a large distance apart so as to obtain a predetermined coupling intensity, resulting in the filter becoming bulkier than in the case of no coupling adjustment holes being formed. This could be avoided by reducing the sizes of the coupling adjustment holes to suppress an increase in the coupling intensity as much as possible. In such a case, however a high degree of working accuracy is required for the necessity of raising the accuracy of the diameters of the coupling adjustment holes, their spacing and the positions of coupling adjusting screws so that the screws may be smoothly inserted into the holes. Especially, it is difficult to make thin and long holes in a hard dielectric material, and this leads to lowered productivity. Moreover, screws that are too thin are not mechanically sturdy and difficult to handle, imposing severe limitations on the reduction of the sizes of the coupling adjustment holes.